18-Geol-A1 Mineralogy and Petrology · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2016-Dec. Closed book; no calculator permitted. Part 1 requires all six 10-mark short-answer questions (60 marks); Part 2 instructs "answer 4 of the 7 ten-mark questions" (40 marks).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (mineral/silicate structural classification, ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, phase diagrams, metamorphic reactions and facies, AFM projections, volcanic processes, ophiolites and oceanic crust, subduction-zone/rift/hotspot melting); Nesse, Introduction to Optical Mineralogy, 4th ed. (index-mineral optics).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Contact metamorphism is localized, thermally-driven metamorphism of the country rock immediately surrounding a hot igneous intrusion (a thermal aureole). Heat conducted outward from the pluton is the dominant agent; confining pressure at typical shallow-to-moderate emplacement depths is modest, and, critically, differential stress is negligible because the country rock is simply baked in place, not tectonically deformed.
Mineral assemblages are zoned outward from the contact, from the highest-grade pyroxene-hornfels facies immediately adjacent to the intrusion, through hornblende-hornfels facies, to lowest-grade albite-epidote-hornfels facies at the aureole's outer edge, defining an index-mineral sequence appropriate to the protolith — for a pelitic protolith, andalusite and cordierite (low-P, high-T index minerals diagnostic of contact rather than regional metamorphism); for a siliceous limestone protolith, decarbonation producing wollastonite ($\text{CaSiO}_3$ from calcite + quartz).
Because there is no differential stress, contact metamorphic rocks (hornfels) are characteristically non-foliated: fine-grained and hard, with a granoblastic (equant, polygonal grains meeting at triple junctions) or decussate (randomly-oriented acicular/platy porphyroblasts, no preferred orientation) fabric. A distinctive spotted texture is common at lower grade, where incipient porphyroblasts appear as darker spots against a finer matrix retaining relict sedimentary texture.
